Re: [RFC PATCH v2 20/27] x86/cet/shstk: ELF header parsing of CET
From: Jann Horn <jannh@google.com>
Date: 2018-07-11 19:37:29
Also in:
linux-api, linux-arch, linux-mm, lkml
On Tue, Jul 10, 2018 at 3:31 PM Yu-cheng Yu [off-list ref] wrote:
Look in .note.gnu.property of an ELF file and check if shadow stack needs to be enabled for the task. Signed-off-by: H.J. Lu <redacted> Signed-off-by: Yu-cheng Yu <redacted>
[...]
quoted hunk ↗ jump to hunk
diff --git a/arch/x86/kernel/elf.c b/arch/x86/kernel/elf.c new file mode 100644 index 000000000000..233f6dad9c1f --- /dev/null +++ b/arch/x86/kernel/elf.c
[...]
+#define NOTE_SIZE_BAD(n, align, max) \ + ((n->n_descsz < 8) || ((n->n_descsz % align) != 0) || \ + (((u8 *)(n + 1) + 4 + n->n_descsz) > (max)))
Please do not compute out-of-bounds pointers and then compare them against an expected maximum pointer. Computing an out-of-bounds pointer is undefined behavior according to the C99 specification, section "6.5.6 Additive operators", paragraph 8; and in this case, n->n_descsz is 32 bits wide, which means that even if the compiler isn't doing anything funny, if you're operating on addresses in the last 4GiB of virtual memory and the pointer wraps around, this could break. In particular, if anyone ever uses this code in a 32-bit kernel, this is going to blow up. Please use size comparisons instead of pointer comparisons.
+
+/*
+ * Go through the property array and look for the one
+ * with pr_type of GNU_PROPERTY_X86_FEATURE_1_AND.
+ */
+static u32 find_x86_feature_1(u8 *buf, u32 size, u32 align)
+{
+ u8 *end = buf + size;
+ u8 *ptr = buf;
+
+ while (1) {
+ u32 pr_type, pr_datasz;
+
+ if ((ptr + 4) >= end)
+ break;Theoretical UB.
+ pr_type = *(u32 *)ptr; + pr_datasz = *(u32 *)(ptr + 4); + ptr += 8; + + if ((ptr + pr_datasz) >= end) + break;
UB, like in NOTE_SIZE_BAD().
+ if (pr_type == GNU_PROPERTY_X86_FEATURE_1_AND && + pr_datasz == 4) + return *(u32 *)ptr; + + ptr += pr_datasz; + } + return 0; +}
[...]
quoted hunk ↗ jump to hunk
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c index 0ac456b52bdd..3395f6a631d5 100644 --- a/fs/binfmt_elf.c +++ b/fs/binfmt_elf.c@@ -1081,6 +1081,22 @@ static int load_elf_binary(struct linux_binprm *bprm) goto out_free_dentry; } +#ifdef CONFIG_ARCH_HAS_PROGRAM_PROPERTIES + + if (interpreter) { + retval = arch_setup_features(&loc->interp_elf_ex, + interp_elf_phdata, + interpreter, true); + } else { + retval = arch_setup_features(&loc->elf_ex, + elf_phdata, + bprm->file, false); + }
So for non-static binaries, the ELF headers of ld.so determine whether CET will be on or off for the entire system, right? Is the intent here that ld.so should start with CET enabled, and then either use the compatibility bitmap or turn CET off at runtime if the executable or one of the libraries doesn't actually work with CET? -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html